1. Technical Field
This disclosure relates to a transconductor circuit, particularly according to the multi-tanh design.
2. Description of the Related Art
One approach that has been used for providing a transconductor circuit, particularly according to the so-called multi-tanh principle, is shown in FIG. 1. This transconductor circuit has a first input node INp for receiving a first input signal and a second input node INn for receiving a second input signal of the transconductor circuit. For example, input signals may be received from an antenna for receiving radio frequency signals, which are processed and provided to a tuner such as a tuner for processing broadcasting signals. A first differential amplifier DA1 is coupled to the first and second input nodes INp, INn, and a second differential amplifier DA2 is also coupled to the first and second input nodes INp, INn. The first differential amplifier DA1 has an offset voltage that is different from the offset voltage of the second differential amplifier DA2. The first differential amplifier DA1 includes a transistor T1 and a transistor T2 each having a control node and a controlled path. In the following, where it is referred to a control node, this is the corresponding gate electrode in the case of a MOS transistor or the base node in the case of a bipolar transistor. Further, where it refers to a controlled path, this shall designate the path between the drain and source node in the case of a MOS transistor or the path between the emitter and the collector in case of a bipolar transistor.
The control node of the first transistor T1 is coupled to the first input node INp and the control node of the second transistor T2 is coupled to the second input node INn, wherein the controlled paths of the transistors T1, T2 are coupled with each other to form the differential amplifier DA1. The second differential amplifier DA2 includes a third transistor T3 and a fourth transistor T4, wherein the control node of transistor T3 is coupled to the first input node INp and the control node of the transistor T4 is coupled to the second input node INn. The controlled paths of the transistors T3, T4 are coupled with each other to form the second differential amplifier DA2, as is commonly known. Thus, the transconductor circuit according to FIG. 1 has a set of differential amplifiers or pairs, which particularly utilize transistors of the bipolar type, whose inputs and outputs are connected in parallel. Both differential amplifiers DA1, DA2 share a common current source S via two resistors R having equal magnitude.
The differential amplifier DA1 has an offset voltage that is different from the offset voltage of the differential amplifier DA2. For example, the offset voltages are generated by emitter area ratios A, as shown in FIG. 1. For example, the emitter area of transistor T3 has the magnitude of A, which may be in principle any number different from 1, whereas the emitter area of the transistor T1 has an emitter area with magnitude 1 (being the reference value with respect to emitter area A). For example, the emitter area of transistor T3 may be ten times greater than the emitter area of transistor T1, so that A=10. In the present case, the transistors T1 to T4 are bipolar transistors.
A transconductor circuit of this type and the known multi-tanh principle are described, for example, in: Barrie Gilbert: “The Multi-tanh Principle: A Tutorial Overview”, IEEE Journal of Solid-State Circuits, VOL. 33, NO. 1, January 1998, which is included herein by reference.
The ratio of the quiescent voltage drop across the resistors R and the offset voltage needs to be kept constant to maintain the excellent linearity of the structure as shown in FIG. 1. Therefore, the transconductance of this structure cannot be controlled or varied via the bias current source S without degrading its linearity.